Radial piston machine

ABSTRACT

A radial piston pump or motor wherein the pistons are connected with the respective shoes by ball and socket joints and the shoes have convex faces travelling along a cylindrical track of the stationary slide block. The concave and convex surfaces of each joint define a hydrostatic relief chamber whose area is at least 70 percent of the cross-sectional area of the respective piston and the radius of curvature of each such concave and convex surface is at least 90 percent of the radius of the respective piston. The outer faces of the shoes define with the inner face of the slide block hydrostatic relief compartments each having an area which is at least 70 percent of the cross-sectional area of the respective piston. The chambers and compartments are connected with the cylinders for the respective pistons by way of channels which are machined into the pistons and shoes so that the pressurized fluid in the chambers and compartments greatly reduces friction between the pistons and shoes as well as between the shoes and the slide block.

United States Patent 1191 Bosch 1 1 RADIAL PISTON MACHINE [75] Inventor: Paul Bosch, Ludwigsburg, Germany [73] Assignee: Robert Bosch G.m.b.H., Stuttgart,

Germany 22 Filed: Apr. 6, 1973 21 Appl, No.: 343,737

FOREIGN PATENTS OR APPLICATIONS 1,228,950 4/1971 United Kingdom 91/488 1 1 May 27, 1975 745,216 2/1931 France 91/497 1,050,498 2/1959 Germany 91/497 764,698 1/1957 United Kingdom 91/497 Primary Examiner-Wil1iam L. Freeh Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT A radial piston pump or motor wherein the pistons are connected with the respective shoes by ball and socket joints and the shoes have convex faces travelling along a cylindrical track of the stationary slide block. The concave and convex surfaces of each joint define a hydrostatic relief chamber whose area is at least 70 percent of the cross-sectional area of the respective piston and the radius of curvature of each such concave and convex surface is at least 90 percent of the radius of the respective piston. The outer faces of the shoes define with the inner face of the slide block hydrostatic relief compartments each having an area which is at least 70 percent of the cross-sectional area of the respective piston. The chambers and compartments are connected with the cylinders for the respective pistons by way of channels which are machined into the pistons and shoes so that the pressurized fluid in the chambers and compartments greatly reduces friction between the pistons and shoes as well as between the shoes and the slide block.

8 Claims, 8 Drawing Figures warm w 2 Y SHEET RADIAL PISTON MACHINE BACKGROUND OF THE INVENTION The invention relates to hydrostatic power units in general, and more particularly to improvements in radial piston machines which can operate as pumps or motors.

It is already known to'provide a radial piston machine with pistons which are coupled to shoes by means of universal joints, particularly ball joints. The shoes travel along the control ring assembly which includes a customary slide block. It is also known to provide those surfaces of the pistons and shoes which transmit radial forces with relief areas in the form of chambers which are connected to the pressure side of the machine.

Swiss Pat. No. 424,487 discloses a radial piston machine wherein the pistons are coupled to the respective shoes by means of ball and socket joints or hinged joints. The soles of the shoes and the surfaces of the joints are provided with relief areas in the form of recesses which are connected to the pressure side of the machine by axial passages provided in the respective pistons. The fluid which fills the recesses serves primarily to lubricate the neighboring faces of the slide block and shoes as well as the neighboring surfaces of the parts of the joints. In addition, the recesses produce a certain hydrostatic relief effect which, however, is practically negligible because the fluid in the recesses can take up only a very small portion of the forces which develop in the working chambers of the pistons. Therefore, the losses due to friction between the control ring assembly and the shoes as well as between the parts of the joints are very high. Some reduction of such friction losses can be achieved at a greatly increased cost by mounting an annular element of the control ring structure for rotation with the cylinder .block; nevertheless, the friction which develops between the parts of joints which couple the pistons to the shoes greatly reduces the output of the machine. This is particularly felt during starting of the machine if the latter is used as a motor.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved radial piston machine wherein the losses due to friction, especially those losses which are due to frictional contact between the surfaces of parts of the joints connecting the pistons to the shoes are much lower than in conventional radial piston machines.

Another object of the invention is to provide a compact radial piston machine wherein the losses in output due to friction which develops between the shoes and the control ring assembly are also lower than in heretofore known machines.

A further object of the invention is to provide a radial piston machine which, particularly when used as a motor, can be started with greater facility than heretofore known radial piston motors.

An additional object of the invention is to provide novel and improved articulate connections between the pistons and shoes in a radial piston pump or motor.

Still another object of the invention is to provide novel and improved connections between the control ring assembly and the shoes in a radial piston pump or motor.

An ancillary object of the invention is to provide a novel and improved control ring assembly for use in a radial piston machine.

The invention is embodied in a radial piston machine which comprises a rotary cylinder block having several radially extending cylinders for pistons which orbit with the cylinder block within a preferably adjustable control ring assembly, a set of shoes each of which is interposed between the head of a piston and the control ring assembly, and a set of ball and socket joints each of which is provided between a piston and the respective shoe. In accordance with a feature of the invention, the convex surface of the ball of each joint has a radius of curvature which is at least 90 percent of the radius of the respective piston and the socket of each joint has a concave surface which is adjacent to and defines with the convex surface of the respective ball a hydrostatic relief chamber whose area (namely, that portion of the chamber which is important for transmission of radial forces) is at least percent of the cross-sectional area of the respective piston. This greatly reduces the friction between the ball and socket of each joint by the simple expedient of connecting each chamber with the pressure side of the machine, e. g., by providing channel means between each chamber and the respective cylinder.

In accordance with another feature of the invention, the neighboring faces of the control ring assembly and of the shoes define hydrostatic relief compartments which communicate with the pressure side of the machine and whose area (again referring to that area which is important for the transmission of radial forces) is at least 70 percent of the cross-sectional area of the respective pistons. Such compartments contribute toa substantial reduction of friction between the shoes and the control ring assembly, especially if the outer faces of the shoes travel along the inner face of a stationary slide block of the control ring assembly.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved hydrostatic power unit itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is an axial sectional view of a radial piston machine which embodies one form of the invention;

FIG. 2 is an enlarged fragmentary sectional view as seen in the direction of arrows from the line [I-II of FIG. 1;

FIG. 3 is a sectional view as seen in the direction of arrows from the line III-II] of FIG. 2, with the piston shown in elevation;

FIG. 4 is a plan view of a shoe substantially as seen in the direction of arrow IV in FIG. 3;

FIG. 5 is a fragmentary sectional view, similar to that of FIG. 2 but showing a portion of a modified coupling between a piston and the corresponding shoe;

FIG. 6 is a fragmentary sectional view, similar to that shown in the upper part of FIG. 2 but illustrating a portion of a modified piston, a portion of the control ring assembly and a modified shoe;

FIG. 7 is a sectional view similar to that of FIG. 2 but showing still another coupling between a piston and the corresponding shoe; and

FIG. 8 is a plan view of the shoe as seen in the direction of arrow VII] in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The radial pistonn machine of FIG. 1 comprises a housing or case 1 including a removable cover 2 which is provided with a central opening 3 for a pintle or valve 4 having in its peripheral surface two angularly offset slots 7, 8 which respectively communicate with axially parallel holes and 6. One of the holes 5,6 admits nonpressurized fluid and the other hole then discharges pressurized fluid when the machine is used as a pump. When the machine is used as a motor, one of the holes 5,6 admits pressurized fluid and the other hole discharges spent fluid. The pintle 4 is held against rotation relative to the cover 2 by a key 4a.

The pintle 4 is surrounded by a rotary cylinder block 9 which is connected with a shaft 11. The cylinder block 9 rotates in part on the pintle 4 and in part on a friction or sleeve bearing 10 which is installed in the bottom wall of the housing 1 opposite the cover 2. The shaft 11 drives the cylinder block 9 when the machine operates as a pump, and the cylinder block 9 drives the shaft 11 when the machine operates as a motor.

The peripheral surface of the cylinder block 9 is provided with several radially inwardly extending cylinders 12 each of which receives a reciprocable piston 13. FIG. 1 merely shows a single cylinder 12 but it will be readily appreciated that the block 9 preferably (and normally) comprises several equidistant cylinders. Each of the pistons 13 is movable in the respective cylinder 12 axially with minimal clearance. The ports 14 of the cylinder block 9 connects successive cylinders 12 first with the slot 7, thereupon with the slot 8, again with the slot 7, and so forth, as soon as the cylinder block 9 rotates in response to admission of pressurized fluid by way of the hole 5 or 6 or in response to starting of an electric motor or another prime mover (not shown) which drives the shaft 11.

The radial piston machine further comprises a control ring assembly including a ring-shaped slide block 17 which surrounds the cylinder block 9 with substantial clearance, and a discrete shoe 15 for each of the pistons 13. Each of the shoes 15 has an arcuate sole which travels along the cylindrical inner face or track 16 of the slide block 17. The latter is a relatively short hollow cylinder and is provided with a radially outwardly extending spindle nut 18 mating with a feed screw 19 which can be rotated by a hand wheel 21. The feed screw 19 is rotatable in a bracket 20 secured to the housing 1. A helical spring 22 reacts against the internal surface of the housing 1 and biases the slide block 17 toward the hand wheel 21. The spring 22 is located diametrically opposite the feed screw 19 and spindle nut 18. It is clear that the hand wheel 21 and the parts 18-20 can be replaced with an automatic slide block control of any known design.

As shown in FIGS. 2 and 3, each piston 13 has a head 23 which resembles the head of a mushroom and constitutes the ball of a ball and socket joint whose socket is formed by the corresponding shoe 15. The concave surface bounding the socket formed by the shoe 15 of FIGS. 2 and 3 is indicated at 24. The corresponding convex surface of the ball 23 is shown at 230.

The coupling means which serves to articulately connect a piston 13 with the associated shoe 15 comprises a pin or post 25 the outer end portion of which is fixedly anchored in the shoe 15 (see FIGS. 2 and 3) and the inner end portion of which can swivel in the piston 13. The outer end portion of the coupling pin 25 is a press-fit in a centrally located bore 26 of the shoe 15. The center of curvature of the surface 23a of the ball 23 forming part of the piston 13 shown in FIG. 2 is indicated at 27; this center of curvature is located radially outwardly of a radially inwardly tapering conical intermediate portion 25a of the coupling pin 25. The inner end portion 29 of the pin 25 resembles a dish and has a concave surface 290 abutting against the convex external surface 30a on the adjacent inner end portion 30 of the piston 13. The dished end portion 29 serves as a means for holding the coupling pin 25 against axial movement relative to the piston 13, and vice versa. The piston 13 has a stepped axial bore including a largerdiameter outer portion 28 which extends radially outwardly all the way to the convex surface 230 and radially inwardly toward but short of the conical coupling pin portion 25a, and a small-diameter portion 280 which receives the conical portion 250 and extends from the larger-diameter portion 28 and all the way to the convex surface 30a.

The center of curvature of the convex surface 30a is the same as that (27) of the convex surface 23a. During assembly, the outer end portion of the coupling pin 25 is forced into bore 26 of the shoe 15 to such an extent that the clearance between the neighboring surfaces 23a, 24 as well as between the neighboring surfaces 30a and 29a is maintained within a predetermined range. The cupped end portion 29 of the pin 25 forms the socket of a second ball and socket joint whose ball is formed by the innermost portion 30 of the piston 13. This joint allows the piston 13 to swivel relative to the coupling pin 25, or vice versa.

The reference character 3] denotes in FIGS. 2 and 3 a hydrostatic relief chamber which is defined by the neighboring surfaces 23a and 24 of the ball and socket joint between the piston 13 and shoe 15. In the illustrated embodiment, the chamber 31 is a recess in the concave surface 24 of the shoe 15. This chamber receives pressurized fluid from the pressure side of the radial piston machine by way of a channel including a portion of the composite bore 28, 28a in the piston 13 and an inclined passage 32 provided in the inner portion of the piston 13 and communicating with the respective port 14 by way of the cylinder 12. The flow of pressurized fluid from the pressure side of the machine to the chamber 31 is practically unrestricted.

The area of that portion of the hydrostatic relief chamber 31 which is important for transmission of radial forces is at least percent and preferably about percent (but not more than 99 percent) of the crosssectional area of the piston 13. When speaking of the area of the hydrostatic relief chamber 31, it is meant to denote that portion of the overall area of such chamber which is hydrostatically effective. The other portions of the chamber 31 are mirror symmetrical to each other with reference to the piston axis so that the pressures of fluid therein balance each other.

The shoe 15 of FIGS. 2 and 3 is further provided with a channel 33 which connects the chamber 31 with a further hydrostatic relief chamber or compartment 34 (hereinafter called compartment to distinguish from the chamber 31). The flow of fluid in the channel 33 is practically unrestricted so that the fluid pressure in the compartment 34 matches that in the chamber 31 and at the pressure side of the radial piston machine.

The compartment 34 is a recess which is provided in the convex outer face a of the shoe 15 and is adjacent to the concave inner face 16 of the slide block 17. The hydrostatically effective area of the compartment 34 (i.e., that portion of the overall area which comes into consideration for the transmission of radial forces) is also at least 70 percent, preferably about 95 percent and not more than 99 percent of the cross-sectional area of the piston 13. Thus, the area of the compartment 34 can match or closely approximates the area of the chamber 31.

The shoe 15 has a hook-shaped or U-shaped outwardly projecting extension 35 (see particularly FIGS. 1 and 3) which extends in the circumferential direction of the cylinder block 9 and into an endless ring-shaped groove 36 provided in an end face of the slide block 17. The inner diameter of the groove 36 is greater than the diameter of the inner face 16 of the slide block 17.

If the radial piston machine operates as a pump, pressurized fluid penetrates from the cylinders 12 into the relief chambers 31 by way of channel means including the respective passages 32 and bores 28,280. Such pressurized fluid also enters the compartments 34 by way of the respective channels 33. Thus, the radialforce-reducing effect (and the resulting decrease of friction) is produced as soon as the slot 7 begins to receive pressurized fluid. It was found that the chambers 31 and compartments 34 reduce the friction between the surfaces'23a,24 and the faces 16, 15a to a very small fraction of that friction which would develop if the chambers 31 and compartments 34 were omitted, or if the effective areas of these chambers were reduced to less than 70 percent of the cross-sectional area of a piston 13. The reduction of friction is especially important during starting of the radial piston machine, particularly if the machine is used as a motor. Moreover, the overall efficiency of the machine is much higher than without such large relief chambers and/or compartments.

If it is necessary to throttle the flow of fluid into the chambers 31 and/or compartments 34, the area of each chamber and compartment is preferably increased so that it exceeds the cross-sectional area of the respective piston 13. Thus, if the passages 32 and/or channels 31 contain flow restrictors, the area of the chambers 31 and/or compartments 34 can be increased to 130-140 percent of the cross-sectional areas of the respective pistons. The arrangement may be such that, when a throttle restricts the flow of fluid in the passage 32 or channel 33 of FIG. 2, the area of the chamber 31 or compartment 34 is increased to such an extent that the metallic surfaces 23a,24 or the metallic faces 1541,16 are completely separated from each other in response to increasing fluid pressure to thus form hydrostatic seals. The extent of separation of the surfaces 230,24 or faces 15a, 16 is such that the pressure of fluid (which is reduced by the flow restrictor in the passage 32 or channel 33) barely carries the outer load.

The extent to which the shoes 15 can move radially inwardly and away from the inner face 16 of the slide block 17 is determined by the extensions 35 of the shoes. Such movement of shoes 15 away from the inner face 16 will take place as a result of friction or while the corresponding pistons 13 perform suction strokes so that the pressure in the chambers 31 and compartments 34 decreases.

ln order to balance the stresses upon the slide block 17, the latter is preferably provided with two grooves (sse the grooves 36 and 36a in FIG. 1). The extensions 35 of the first, third, etc. shoes 15 slide in the groove 36 and the extensions 35 of the second, fourth, etc. shoes 15 slide in the groove 36a.

When the flow of pressurized fluid from the pressure side of the machine to the chambers 31 and compartments 34 is not throttled, the area of each chamber 31 and each compartment 34 should not exceed 99 percent of the cross-sectional area of the respective piston because, otherwise, the surfaces 24 of the shoes 15 would be lifted off the surfaces 23a of the pistons 13 and the faces 15a of the shoes 15 would be lifted off the inner face 16. However, and as mentioned before, the placing of throttles into the passages 32 and/or channels 33 brings about a reduction of fluid pressure in the chambers 31 and/or compartments 34 so that their areas can be increased to exceed (e.g., to correspond to l30-l40 percent of) the cross-sectional areas of the pistons. Thus, the areas of the chambers 31 and compartments 34 can exceed the cross-sectional areas of the pistons 13 when the pressure in such chambers and compartments is less than in the corresponding cylinders 12. The relationship between the flow restrictors and the areas of the chambers 31 and compartments 34 is selected with a view to insure that the fluid in such chambers and compartments can take up substantial radial stresses but is still unable to bring about excessive separation of surfaces 23a,24 and faces 150,16.

An important advantage of relatively large hydrostatic relief chambers and compartments is that the fluid therein can take up a very large percentage of radial forces to thus greatly reduce mechanical friction between the pistons and shoes as well as between the shoes and the control ring assembly. The friction between mechanical parts is replaced to a large extent by friction between mechanical parts and a body of liquid. The forming of chambers whose area is at least percent (and preferably at least percent) of the cross sectional area of the pistons is made possible by selecting the radius of curvature of each concave and convex surface of each joint between a piston and the respective shoe in such a way that the radius of curvature is at least 90 percent of the radius of the piston.

ln heretofore known radial piston machines, the area of a hydrostatic relief chamber does not exceed 40 percent of the cross-sectional area of the respective piston. This is believed to be attributable to the fact that, in accordance with the presently prevailing opinion of people skilled in this art, a further increase in the effective area of such relief chambers would result in excessive leakage of pressurized fluid.

FIG. 5 illustrates a portion of a modified piston 40 which forms with the cylinder block 9 a flow restrictor 41 so that the flow of pressurized fluid from the cylinder 12 into the larger-diameter portion 28 of the axial bore in the piston 40 is throttled upstream of a passage 43. The compartment 34 (not shown) is assumed to receive fluid from the respective chamber 31 (not shown) in the same way as shown in FIG. 2. Thus, the flow restrictor 41 throttles the flow of fluid from the cylinder 12 to the hydrostatic relief chamber as well as to the hydrostatic relief compartment. In the embodiment of FIG. 5, the area of each chamber and each compartment can be increased to exceed the crosssectional area of the respective piston. The illustrated flow restrictor 41 includes a narrow .clearance or gap between an external ring-shaped projection of the piston 40 and the internal surface of the block 9. The periphery of the piston 40 is further provided with a ringshaped recess or groove 42 which receives fluid flowing through the flow restrictor 41 and communicates with the passage 43, i.e., with the respective chamber 31 and compartment 34.

The smaller-diameter portion 280 of the axial bore in the piston 40 is practically sealed from the largerdiameter portion 28 by a cylindrical portion of the coupling pin 25 immediately outwardly of the conical portion 25a.

In all other respects, the construction and operation of the radial piston machine which embodies the structure of FIG. are identical with the construction and operation of the machine shown in FIGS. 1 to 4.

FIG. 6 shows a portion of a third radial piston machine wherein the piston 46 has a ball 47 provided with a convex surface 47a adjacent to the concave surface 48 in the socket of the shoe The relief chamber is shown at 49; the width of this chamber (as seen in the radial direction of the cylinder block) increases from the marginal portion toward the center of the convex surface 47a. This is achieved by providing the ball 47 with a convex surface 47a whose radius of curvature is somewhat greater than the radius of curvature of the concave surface 48. Thus, one can say that the surfaces 47a and 48 contact each other only along a line, namely, along a circle whose diameter is shown at D.

The relief compartment 50 between the inner face 16 of the slide block 17 and the outer face 150' of the shoe 15 is formed in similar fashion. Thus, the radius of curvature of the outer face 15a is greater than the radius of the inner face 16 whereby the width of the compartment 50 decreases from its central portion 51b toward its front and rear end portions 51,51a, i.e., in and counter to the direction (arrow E) of orbital movement of the shoe 15'. The points of contact between the leading and trailing edges of the face 15a and the inner face 16 are adjacent to the compartment portions 51 and 51a. The manner in which the chamber 49 and compartment 50 receive pressurized fluid is preferably the same as described in connection with FIGS. 1 to 4.

If the pressure in the chamber 49 and compartment 50 happens to rise for a short interval of time to such a value that the surfaces 470, 48 and the faces 15a, 16 are separated from each other, fluid can leak from the chamber 49 and compartment 50 whereby its pressure drops. A balance is achieved when the pressure of fluid in the chamber 49 and compartment 50 matches the external load. At such time, the surfaces 470, 48 and the faces 15a, 16 are completely separated from each other by thin hydrostatically obtained films of liquid.

An advantage of the embodiment which is shown in FIG. 6 is that the flow restricting means can be omitted. Such flow restricting means could be clogged by foreign matter to thus prevent satisfactory flow of pressurized fluid into the chambers and compartments.

FIGS. 7 and 8 illustrate a fourth embodiment of the radial piston machine. The piston 55 forms the socket (rather than the ball) of the ball and socket joint. Thus,

the concave surface 56 of the piston 55 is adjacent to the convex surface 59a of a ball 59 which forms part of the shoe 57. The piston wall which is adjacent to the maximumdiameter (outer) end of the socket bounded by the concave surface 56 is rather thin. The ball 59 is provided on an extension or shank 58 of the shoe 57 whose outer face 570 defines with the inner face 16 of the slide block 17 a compartment 50A which is identical with the similarly referenced compartment of the structure shown in FIG. 6.

The coupling means between the shoe 57 and piston 55 comprises a pin 60 which extends with clearance axially into an axial bore of the shank 58 and has an inner end portion tightly fitted into an axial bore 61 of the piston 55. The outer end portion 62 of the coupling pin 60 forms a ball whose convex surface 62a is adjacent to a concave internal surface 5% provided in the ball 59 of the shoe 57. The parts 59 and 62 form a ball and socket joint which allows the shoe 57 to swivel relative to the piston 55 or vice versa.

The reference character 63 denotes the common center of curvature of the convex surfaces 59a, 62a and concave surfaces 56, 59b.

The hydrostatic relief chamber 80 between the neighboring surfaces 590, 56 of the ball and socket joint between the piston 55 and shoe 57 extends to an annular groove 64 which is machined into the ball 59 close to its equator. This insures that the area of the chamber 80 is not less than percent and preferably not less than 90 percent of the cross-sectional area of the piston 55. The area of the relief compartment 50A may equal or exceeds the area of the chamber 80. The compartment 50A is similar to the compartment 50 of FIG. 6.

The chamber receives fluid by way of a channel including a passage 81 in the piston 55. The compartment 50A receives fluid by way of an axial bore 82 in the coupling pin 60 and the axial bore 83 in the shoe 57.

It will be noted that the ball of the ball and socket joint between a piston and the respective shoe can be provided on the piston (FIGS. 1-6) or on the shoe (FIGS. 8-9). It is further clear that the groove or grooves 36,360 in the slide block can be replaced with rings extending into arcuate grooves provided in the adjacent faces of the shoes. Still further, the features of the embodiments which are respectively shown in FIGS. 1-4, 5, 6 and 7-8 can be combined in any desired way, for example, by replacing the compartment 34 of FIG. 2 with a compartment 50 of the type shown in FIG. 6.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended:

1. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; and a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature exceeds the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross sectional area of the respective piston.

2. In a radial piston machine a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders and each provided with an axial bore; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe, each of said coupling means comprising an elongated coupling member in the form of a pin extending with radial clearance through the axial bore in the respective piston and having an outer end portion having a press fit in the central bore of the respective shoe, each of said pistons having an end portion remote from the respective ball and socket joint and each of said coupling members having a second end portion overlying the inner end portion of the respective piston.

3. In a radial piston machine, a combination compris ing a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders and each provided with an axial bore; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe, each of said coupling means comprising an elongated coupling member extending with radial clearance through the axial bore in the respective piston and having an outer end portion rigid with the respective shoe, each of the pistons having an end portion remote from the respective ball and socket joint and each of said coupling members having a second end portion overlying the inner end portion of the respective piston, the second end portion of each of said coupling members having a concave surface whose center of curvature coincides with the center of curvature of the convex surface of the respective ball, the inner end portion of each of said pistons having a convex surface which is adjacent to the concave surface of the second end portion of the respective coupling member so that each of said inner end portions defines with the respective second end portion a ball and socket joint.

4. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons, said control ring assembly comprising a stationary slide block having an inner face constituting a track for said shoes. two end faces and a ring-shaped groove in at least one of said end faces, each of said shoes comprising an extension projecting into said groove; and a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective hall and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston.

5. A combination as defined in claim 4, wherein the smallest diameter of said groove exceeds the diameter of said track.

6. In a radial piston machine. a combination compris ing a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe. each of said coupling means comprising an elongated coupling member having a first end portion rigid with the respective piston and a second end portion extending with clearance into a central bore of the respective shoe.

7. A combination as defined in claim 6, wherein said first end portion of each-coupling member is a press fit in an axial bore of the respective piston and the second end portion of each coupling member has a convex surface abutting against a concave surface in the interior of the respective shoe.

8. A combination as defined in claim 7, wherein the center of curvature of the convex and concave surfaces of each of said joints coincides with the center of curvature of the convex surface of the second end portion of the respective coupling member and with the center of curvature of the concave internal surface of the respective shoe. 

1. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; and a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature exceeds the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross sectional area of the respective piston.
 2. In a radial piston machine a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders and each provided with an axial bore; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe, each of said coupling means comprising an elongated coupling member in the form of a pin extending with radial clearance through the axial bore in the respective piston and having an outer end portion having a press fit in the central bore of the respective shoe, each of said pistons having an end portion remote from the respective ball and socket joint and each of said coupling members having a second end portion overlying the inner end portion of the respective piston.
 3. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders and each Provided with an axial bore; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe, each of said coupling means comprising an elongated coupling member extending with radial clearance through the axial bore in the respective piston and having an outer end portion rigid with the respective shoe, each of the pistons having an end portion remote from the respective ball and socket joint and each of said coupling members having a second end portion overlying the inner end portion of the respective piston, the second end portion of each of said coupling members having a concave surface whose center of curvature coincides with the center of curvature of the convex surface of the respective ball, the inner end portion of each of said pistons having a convex surface which is adjacent to the concave surface of the second end portion of the respective coupling member so that each of said inner end portions defines with the respective second end portion a ball and socket joint.
 4. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons, said control ring assembly comprising a stationary slide block having an inner face constituting a track for said shoes, two end faces and a ring-shaped groove in at least one of said end faces, each of said shoes comprising an extension projecting into said groove; and a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston.
 5. A combination as defined in claim 4, wherein the smallest diameter of said groove exceeds the diameter of said track.
 6. In a radial piston machine, a combination comprising a rotary cylinder block having substantially radially extending cylinders; pistons reciprocably extending into said cylinders; a control ring assembly spacedly surrounding said pistons; shoes interposed between said assembly and said pistons; a ball and socket joint articulately connecting each of said shoes with one of said pistons, the ball of each of said joints having a convex surface whose radius of curvature is at least 90 percent of the radius of the respective piston and the socket of each of said joints having a concave surface adjacent to the convex surface of the respective ball and defining therewith a hydrostatic relief chamber which is in communication with the pressure side of said machine and whose area is at least 90 percent of the cross-sectional area of the respective piston; and a plurality of coupling means each arranged to articulately connect one of said pistons with the respective shoe, each of said coupling means comprising an elongated cOupling member having a first end portion rigid with the respective piston and a second end portion extending with clearance into a central bore of the respective shoe.
 7. A combination as defined in claim 6, wherein said first end portion of each coupling member is a press fit in an axial bore of the respective piston and the second end portion of each coupling member has a convex surface abutting against a concave surface in the interior of the respective shoe.
 8. A combination as defined in claim 7, wherein the center of curvature of the convex and concave surfaces of each of said joints coincides with the center of curvature of the convex surface of the second end portion of the respective coupling member and with the center of curvature of the concave internal surface of the respective shoe. 